The representation of space is critical for the coordination of many forms of human behavior. The overarching question addressed in this project is, what psychological and neural mechanisms underlie the process of spatial representation. Moreover, how this derived spatial code is exploited in the service of action, such as moving one's eyes or hands to the location of the desired target. While there is general acceptance in the animal and human literature that the coding of spatial position is neurally instantiated in the parietal lobe, there is little agreement on the frame of reference or coordinate system with respect to which spatial locations are defined. While some recent progress has been made in clarifying this issue, there remain two particularly contentious issues, one concerning the involvement of an object-centered reference frame in the coding of spatial position during perception and the second concerning the consequences of a deficit in spatial coding for action. Two major methodologies will be used to address these issues. The primary method involves the detailed analysis of the behavior (with reaction time, parameters of eye movements and hand movements as dependent measures) of neurological patients who, following a lesion to the parietal lobe, have a selective deficit in spatial behavior (known as hemispatial neglect). The complementary method employs functional magnetic resonance imaging (fMRI) to examine both the neural site and time course of the processes involved in spatial representation in normal and brain-damaged subjects. The first series of experiments compares the performance of neglect patients on tasks designed to evaluate how different spatial reference frames (particularly one that is object-centered) determine what is neglected. The second set of experiments explores the spatial reference frames in output modalities in experiments requiring eye movements and/or visually guided reaching. Individual spatial frames of reference will be explored to examine their unique contribution to spatial behavior. Studies using fMRI are interleaved with the neuropsychological studies to provide converging evidence. The ease with which coordinated spatial behavior is executed belies the underlying complexity of the process. This project may provide important insights into two central issues: the process of spatial representation in normal subjects and its breakdown in humans with lesions to the parietal cortex. An improved understanding of what gives rise to neglect might lead to better diagnostic and remediation procedures for this fairly common neurobehavioral disorder. These findings will also provide a crucial bridge to the important lesion and single unit studies in nonhuman primates and will constrain theoretical models of spatial cognition.